A conventional sealed lead-acid storage battery is constructed as shown in FIG. 4 which is used as a power source for a portable apparatus or the like. In FIG. 4, is illustrated a casing made of a thermoplastic resin 1 for receiving a group of electrodes 6 composed of a positive electrode 2, negative electrode 3, and separator 4. An opening portion of a casing 1 surrounding the periphery of the group of the electrodes 6 is hermetically sealed when the opening portion is thermally bonded to a periphery of a metal plate 7 for a battery casing, an inner surface of which metal plate has been already laminated with a polyethylene film or the like. Numeral 5 denotes a pole bar portion, and numeral 8 denotes a vent valve formed at the casing 1 and having the function of opening the valve with the increment of the inner pressure of the battery. The method for completely sealing the pole bar portion 5 will be described hereinbelow. One method is that an adhesive such as an epoxy resin is filled and hardened. The other method is that a sheet of lead alloy previously coated with the epoxy resin or a polyethylene resin is welded to a protruding portion of the electrode by TIG welding or the like to form a pole bar, and the resulting pole bar is thermally bonded between the casing and the metal plate for a battery casing.
By the structure described above, it was possible to make a sealed lead-acid storage battery, which is thin, i.e. less than 10 mm, and has a high volumetric efficiency. As shown in FIGS. 5 and 6, the vent valve 8 of this type of sealed lead-acid storage battery is constructed as described hereinbelow. A concave portion 14 having a flat bottom as a valve seat 15 is provided at the center portion of the thick end of the casing 1 forming a battery casing, and a vent port 13 is formed at the center of the vent seat 15. Moreover, a rubber sheet flat valve 12 for sealing the vent port 13 and a cushion member 11 resiliently compressing the rubber sheet flat valve 12 are provided, and there is mounted a cushion pressing plate 10, which holds the top of the rubber sheet flat valve 12 and the top of the cushion member 11 and closes the concave portion 14. The cushion pressing plate 10 is secured to the casing 1 forming the battery casing by ultrasonic bonding.
In such a conventional structure, the hardening of adhesive usually requires approximately one hour at 60.degree. C. because an adhesive such as an epoxy resin or the like is used for sealing of the pole bar portion 5. As a result, it is impossible to improve productivity in the production process. In the method wherein a sheet of lead alloy coated with the epoxy resin or the polyethylene resin is welded to the protruding portion, it was difficult to provide an efficient assembling line for sealed lead-acid storage batteries which can operate at high speeds and low costs, since there is a problem in reliability of the welding and cost.
On the other hand, as to the vent valve 8, there are problems described below in the above-mentioned structure: (1) the cost rises since the number of parts to be used is large, (2) hereafter, there is a tendency for this type of sealed lead-acid storage battery to be miniaturized, and subsequently the parts to be used are miniaturized, so that the operation efficiency becomes worse, (3) the number of operations is large in the process of ultrasonic bonding operation or the installation cost for bonding is high.